1. Field of the Invention
The present invention relates to a liquid developer.
2. Description of the Related Art
In a general dry toner (including a resin and a color material and also referred to as “dry developer”) used in an electrophotographic image forming apparatus, 10 parts by mass or less of the color material (pigment) is included in 100 parts by mass of the resin. This ratio is determined by a relation of image density relative to a particle size of a toner particle (i.e., image film thickness). On the other hand, in a liquid developer (also referred to as “wet developer”), a toner particle has a particle size smaller than that of the dry toner. Hence, required image density cannot be secured unless a ratio of the color material is increased.
Of such liquid developers, for example, a black developer generally employs a carbon black as a pigment. In order to secure image density, approximately 15 parts by mass or more of the carbon black needs to be included relative to 100 parts by mass of the resin. This ratio of the carbon black generally differs depending on the particle size of the toner particle.
Recently, in order to achieve high image quality, prevention of strike-through, and low cost, a demand arises in decreasing an amount of liquid developer (toner particles) adhered onto a recording material. Accordingly, the film thickness of the image tends to be decreased. However, image density needs to be satisfied even under such a condition. Hence, it is necessary to increase the content of the pigment in the toner particle.
Meanwhile, when the content of the pigment is increased in the toner particle, the amount of resin (also referred to as “binder resin”) is relatively decreased to result in decrease of fixing strength onto the recording material. In addition, such an increased content of the pigment in the toner particle results in deterioration of dispersion property of the pigment in the resin. This leads to deterioration of electric property (transferring property) of the toner particle.
Under such a circumstance, it has been proposed to use a specific dispersant so as to improve the dispersion property of the toner particle in the liquid developer (Japanese Laid-Open Patent Publication No. 2011-027845; Japanese Laid-Open Patent Publication No. 2011-043650), or it has been proposed to use a specific dispersant so as to improve the dispersion property of the pigment in the toner particle (Japanese Laid-Open Patent Publication No. 2001-159834).